Given that Palestinian nationalists intend to create an Islamic theocracy that goes against everything these left-wing groups claim to represent, why would they ever support them? They are effectively condoning the stoning of homosexuals, genocide against Jews and Christians, women would be second-class citizens, police brutality would be commonplace and the sacrifice of almost every civil and human right these groups hold dear. Not only that, but a Palestinian state would be fundamentally anti-Western, unstable (see Fatah-Hamas conflict) and a state sponsor of terrorism.
How could anyone who claims to stand for "humanist values" or "freedom" possibly support Palestinian nationalism at this time? Surely it isn't worth jeopardizing every freedom you believe in, just so you can say "I'm too cool for Israel." Why not use this energy to support a legitimate cause instead, like Kurdish self-determination?
Note: I'm not saying that all Palestinians believe in these things, but the majority of them and their leaders do. I'm also not saying that there couldn't be a two-state solution far into the future (once Palestinian terror groups abandon violence). The Palestinians just aren't ready for statehood at this time.